Alkynyl groups addition reactions

Introduction of a trimethylsilyl group at the terminal alkynyl position of the foregoing chiral mesylates reverses the regiochemistry of the addition reactions (Table 9.35) [50]. In these systems, the allenyl adducts are strongly favored in additions that are highly diastereo- and enantioselective. 

The total of the reactions 2-5 results in reaction 1. The oxidative addition and formation of copper acetylide are well known. The intermetal transfer of the alkynyl group from copper acetylide (e.g., from Cu(I) to Pd(II) [8]) has been revealed by organometallic chemistry [8-11]. 

A new route to bromopyrroles was developed. It depends on addition of HBr to A-protected y-aminoynones. When applied to alkynyl ketones, 2-aryl or 2-alkyl 4-bromopyrroles are formed. 2-Alkyl or 2-aryl 3-bromopyrroles can be obtained from acetals of V-aminoynals. The ketones are made from A -protected propargylamines by ( -acylation. The acetals are made from 3,3-diethoxypropyne by addition to an aldehyde followed by introduction of the amino group by reaction with phthalimide under Mitsunobu conditions. <95S276>... 

An interesting sequence based on an intermolecular Michael addition and a subsequent transition metal-catalyzed carbocyclization was recently explored. Much of the development of this strategy relies on recent studies related to the intramolecular carbometalation reaction of stabilized carbanions bearing an unactivated alkynyl group. Several transition metal complexes such as Cu [52], Pd [53], Ti [54], Zn [55], Co [56] and Sn [57] have been reported to catalyze this reaction (Scheme 17). 

The dual ability of carbonyl, alkenyl, and alkynyl groups to act as radical precursors as well as radical acceptors is of great importance for synthetic applications, since it would diversify the formation of carbon-carbon bonds by radical reactions. Each functional group has unique characteristics, which permit the generation of radicals from multiple bonds or the addition of radicals to multiple bonds in a selective manner. 

Pfinutry secondary, and even tertiary alkyl groups undergo the additJoc reaction, m do aryl and alkenyl giHiupa. Alkynyl groups, however, rend poorly in che- cor i ugnte addition process. 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

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